The Black-headed Gull removes the empty egg shell shortly after the chick has hatched. The present paper describes some experiments on the function of this response, and on the stimuli eliciting it. Carrion Crows and Herring Gulls find white eggs more readily than normal gulls' eggs; it is concluded that the natural colours of the eggs afford a certain degree of cryptic protection. When normal eggs are given an egg shell at 15 cm. distance their vulnerability is greatly increased; this "betrayal effect" decreases rapidly with increased distance between egg and shell. We therefore conclude that egg shell removal helps to protect the brood from predators. As reported by C. BEER (1960) the Black-headed Gull removes a surprisingly wide range of objects from the nest. Large scale tests with egg shell dummies in which colour, shape, size and distance from the nest were varied showed that objects of all colours are carried but that "khaki" (the normal ground colour of the egg) and white are particularly stimulating, while green elicits very few responses. Egg shells elicit more responses than cylindrical rings of the same colour, and these are responded to better than "angles". Size can be varied within wide limits; very large rings elicit fear which interferes with removal. Various other indications are mentioned which show that the score as obtained in the mass tests does not accurately reflect the responsiveness of the reaction itself but rather the result of its interaction with other behaviour tendencies. The eliciting effect decreases rapidly with increasing distance. On the whole, the gulls' response is very well adapted to its main function of selectively removing the empty shell, but the relatively high scores for objects which have very little resemblance to egg shells suggest that it is adapted to the removal of any object which might make the brood more conspicuous. A pilot test showed that gulls which have incubated black eggs respond better to black egg shell dummies than normal gulls. The lack of promptness of the response as compared with non-colonial waders (Ringed Plover and Oystercatcher) is adaptive, since it tends to reduce predation by other Black-headed Gulls, which are shown to prey selectively on wet chicks. A hitherto unrecognised function of territory is suggested. In a discussion of the entire anti-predator system of the Black-headed Gull its complexity and its compromise character are stressed: the safety demands of the individual clash with those of the brood; there are conflicts between the several safety devices which each benefit the brood; and there are clashes between the ideal safety measures required by each type of predator.